Known as an example of conventional methods for manufacturing an interposer is one disclosed in Patent Literature 1. First, in this method for manufacturing an interposer, a sheet-like object to be processed is irradiated with a laser light, so as to form a precursory hole in the object. Subsequently, the object 1 is etched, so as to enlarge the precursory hole, thereby forming a through hole. Then, a conductor is inserted into the through hole, so as to form a through-hole electrode, thereby manufacturing an interposer.